The story
First sense drives from the UK practice of flashing lights at another car to indicate they can proceed, + cash. Second sense derives from the flash of the camera's light.
Etymology from Wiktionary, CC BY-SA 4.0.
flash for cash
Today it means: Pertaining to a scam where a motorist flashes their headlights to indicate to another driver to proceed against normal give-way rules, then runs into them and sues for damages. Full definition →